What Causes Spitting Bubbles And Breathing Heavily When Asleep As He Has Asthma?

My son was sent home due to his asthma acting up, his attacks always start with a cough. They gave him his Albuterol inhaler and when I got him home, I gave him a Pulmicort nebulizer treatment. He is now asleep, breathing heavy still and has foamy looking spit bubbles at his mouth. Is that something to worry about or does it just have to do with him breathing with his mouth partially open?